Numerical Investigation Of Three-dimensional Separation Flow Field Of Single Expansion Ramp Nozzle

Over-expansion flow separation is a hot topic in nozzle research,but a large part of the research is based on quasi-two-dimensional(2D)flow field.The jet of a nozzle is essentially a three-dimensional(3D)behavior,and in the 3D separated flow field,not only the common central separation exists,but also the sidewall separation as well as the corner separation,which are the main source of the three-dimensionality of the flow field.Flow separation is characterized by complex shock wave/boundary layer interactions(SWBLI).Different separation patterns have different flow fields with 3D shock wave structures,and transitions occur between the various separation patterns.Therefore,the present study focuses on single expansion ramp nozzle(SERN),mainly using numerical simulations to investigate the coupling relationship between the separation regions,the 3D shock wave structures of different separation patterns and the transition mechanisms between different separation patterns.Firstly,four turbulence models were used to simulate the typical flow field structure of the SERN,and the boundary conditions were kept consistent in the calculation.Subsequently,the calculated results were compared with the experimental data,the differences for the calculated results of different models were analyzed.The results show that RNG k-ε model has the strongest ability to simulate the separated flow field in the SERN,the wall pressure distribution and the numerical schlieren of the flow field calculated by this model are in good agreement with the experimental data,so RNG k-εturbulence model is used for subsequent studies.Secondly,3D steady simulations with different nozzle pressure ratio(NPR)were carried out to obtain 3D flow field data of various separation patterns,which were displayed through various visualization methods.The data showed that flow separation also occurs at the sidewall and corner after considering the effect of sidewalls,which causes the structure of the separated flow field to change significantly in the spanwise direction.The aspect ratio of the nozzle is relatively large,and the 3D effect of the flow field is limited,so the flow field structure of the symmetry plane does not differ much from the 2D simulation results.However,the effect of the sidewall on the overall flow field is limited,and the flow field structure of the nozzle symmetry plane does not differ much from the two-dimensional simulation results.In the three-dimensional separation flow field,the flow separation at sidewalls may induce a lateral λ shock wave.Due to the deceleration effects of both side walls on the corner flow,the low-energy gas accumulated here will always separate first.Finally,3D unsteady simulations of the SERN were carried out to obtain data on the evolution of the 3D flow field structure during nozzle start-up and shutdown.The variation of the performance parameters obtained from the 3D simulation is more moderate compared to the 2D,because the structure of the 3D flow field does not change synchronously in the spanwise direction,there is a definite sequential relationship.The analyses show that the flow field near the sidewall dominates the variation of the flow field structure during the shutdown process,while the start-up process is dominated by the flow field near the symmetry plane.Regardless of the start-up or shutdown process,the transition of the separation patterns actually drives the flow field toward a stronger "two-dimensional" flow field.